Andy Beal details a very interesting action Yahoo! seems to be doing when people upgrade the Yahoo Messenger. Not only do you get the Messenger updated but you also get the new IE 7 with most of the settings set to Yahoo as the default.
Now that is some good black hat!
The article about hijacking the search and other features in IE7 (which is still a little buggy) is well documented with screen captures.
Good catch Andy!